The light forming the image that is focused onto an imaging device has an inherent efficiency for detection. The photon flux through an aperture falls onto a sensor surface and is converted into an electrical signal that is stored as one component of a pixel of a digital image. Each pixel of the image is typically composed of either of three individual photodetectors corresponding to a color domain, having three of the fundamental colors, e.g., red, green, blue (RGB) or the complimentary color domain having cyan, magenta, yellow (CMY). The intensity of the electrical signal is roughly proportional to the quantity of photons sensed at each photodetector. A lens, or microlens, is typically used to focus the incoming light into the active area of each photodetector. Larger numbers of photons provide a better signal-to-noise ratio from the device. The microlens may be formed from any set of process steps which generate a convex lens shape in a material having a relatively high refractive index, e.g., greater than 1.5, above the photodetector.
Typical formation processes incorporate resist reflow, or dielectric material reflow, techniques. These processes require precise control of the reflow temperature, duration, thickness of the material, etc. Many factors can cause variation in the ultimate shape of the lens. Lens shape repeatability and aberrations caused by distortions are common problems using such techniques.
Another problem with lens stack design is that an optimum design for one color may not be quite right for another color. The reason for this is dispersion in a typical lens material is a function of light wavelength, as shown in FIG. 1, which depicts the index of refraction, n, for a lens material, wherein n varies with the wavelength.
Titanium dioxide (TiO2) is a material frequently incorporated into microlenses. The refractive index for a TiO2 lens varies with wavelength, and the difference can be as much as 0.1 between the blue and the red regions. In contrast, silicon dioxide, which is generally used to encase a TiO2 lens, has refractive index of 1.47 with an index of refraction range of less than 0.01 between the blue and red regions. The lens stack focal point depth is different for blue and red, perhaps by as much as 0.25 microns. TiO2 material is known to have a high refractive index, while being transparent in the visible range for maximum light transmission. Other high refractive index transparent materials exist wherein similar dispersion of n is seen.
U.S. Pat. No. 5,324,623, to Tsumori, granted Jun. 28, 1994, for Microlens forming method, describes a method of forming a microlens made of a thermoplastic resin on a solid-state imaging device, wherein the surface of an imaging device is patterned, and filled with the thermoplastic resin, which is then thermally deformed.
U.S. Pat. No. 6,163,407, to Okazaki et al., granted Dec. 19, 2000, for Microlens array and method of forming same and solid-state image pickup device and method of manufacturing same, describes a pattern for a microlens array and a material layer of the microlens array, which are simultaneously etched under a condition by which planar patterns transferred from the resist to the material layer are larger than planar patterns of the resist. The spacing between microlenses can be made narrower than the spacing between the planar patterns of the resist.
U.S. Pat. No. 6,417,022 B1 to Hsiao et al., granted Jul. 9, 2002, for Method for making long focal length micro-lens for color filters, describes a method including providing a layer of micro-lens material, which is spin coated on a color filter, patterned by a photolithographic method into a number of discrete regions for each micro-lens with a pre-set spacing therein between. The discrete regions allow a smaller volume of micro-lens material to be used for forming the micro-lens in a subsequent reflow process.
U.S. Pat. No. 6,495,813 B1 to Fan et al., granted Dec. 17, 2002, for Multi-microlens design for semiconductor imaging devices to increase light collection efficiency in the color filter process, describes a microelectronic fabrication methods for forming planar array multi-microlenses comprised of elements consisting of lens-pairs, integrated with color-filters, and compatible with CMOS high-volume manufacturing are taught.